Line to travelling wave tube coupling



April 1963 KARL-BERNHARD NICLAS 3,

LINE T0 TRAVELLING WAVE TUBE COUPLING Filed Oct. 20, 1958 A\ IL Q 7 I E vl I| 1 3 6 Q l i I t f 4 L I v INVENTOR: Karl-Bernhard Niclus PATENT AGENT United States Patent 3,087,089 LINE T0 TRAVELLING WAVE TUBE COUPLING Karl-Bernhard Niclas, Ulm (Danube), Germany, assiguor to Telefunken G.m.b.H., Berlin, Germany Filed Oct. 20, 1958, Ser. No. 768,241 Claims priority, application Germany Oct. 26, 1957 4 Claims. (Cl. SIS-39.3)

The present invention relates to apparatus for coupling a high frequency line, particularly, to a helical delay line of an electrical discharge device, such as a traveling wave tube. The vacuum envelop of this tube is of dielectric material and is surrounded -'by a metal tube connected to the high frequency line so as to conduct high frequency energy and carrying the focussing means (not shown) which focus the electron beam as it travels through the delay line, this tube being hereinafter referred to, in the specification, simply as the focussing tube. The coupling end of the delay line of this travelling wave tube is provided with at least one cup-shaped sleeve, located within the vacuum tube, said sleeve forming, together with the metal tube outside of the vacuum envelop, a coaxial line of A wave length, said coaxial line providing a high frequency short-circuit across the end of the cupshaped sleeve, the wave length referred to being that of a wave whose frequency is that to which the system is tuned.

In case of coupling of travelling wave tubes to a hollow guide or coaxial cable, it has been conventional to extend the hollow guide or coaxial line into the tube. However, since it is technically difiicult to provide this coupling by conductive contact of the parts, this structure has been designed in such a way that, with the aid of a )\/4 cup-shaped wave length sleeve within the tube and a metal focussing tube therearound, there are obtained no-load coaxial lines which transform or shift the short circuits to the joints of the connection. It has been known that the matching of a tube to a highfrequency line for definite frequencies is sharply impaired when the operating wave length of the travelling wave tube is below the cut-off wave length of the H mode in a coaxial cable. This is due to the fact that the M4 condition exists for the conventional coaxial cable mode, but not for the H mode for the same operating frequency. If a wave of the H mode is propagated between the two high frequency conductors of the 7\/4 arrangement, the desired high frequency short circuit at the operating frequency is no longer present between the two conductors and a considerable mismatch is obtained which results in inferior eificiency of the entire system.

It has been known to provide so-called M4 sleeves at the ends of the delay line for the purpose of coupling the high frequency lines to the output of a traveling wave tube. Furthermore, it has been known to provide a number of slots, such as longitudinal slots, in these M4 sleeves in the axial direction. In addition to this, it has been known to use the abovementioned focussing tube as the outer conductor of the 4 coupling system.

However, none of these known systems is capable of safely preventing the propagation of the H mode, particularly, if the apparatus or system is operated at very high frequencies.

Thorough investigation of the H mode showed that it is necessary to interrupt the axial current path formed at the input of the M 4 sleeves in order to avoid this mode.

Accordingly, it is an object of the present invention to provide several circumferentially disposed slots in the part of the wall of the M4 sleeve opposite or adjacent the metal focussing tube.

Further investigations have shown that with fixed dimensions of the travelling wave tube, a certain diameter ice of the focussing tube can be ascertained at which the cut-off wave length of the H mode is the lowest. In other words, it is possible to define an optimum focussing tube diameter for the shortest cut-off wave length, said diameter being calculated with fixed travelling wave tube dimensions with the aid of the following approximate equation:

1 1n Dz/D 1 E, lnD /D 2m D /D, 1),

wherein:

111 represents the natural logarithm D is the inner diameter of the dielectric vacuum envelop D is the outer diameter of the dielectric vacuum envelop D, is the inner diameter of the focussing tube D, is the outer diameter of the h/ 4 sleeve, and

E is the dielectric coefficient of the vacuum tube.

This equation is obtained by the following consideration.

The cut-off wave length of the H mode is obtained in first approximation by:

wherein The reason for the occurrence of the minimum of the cut-off wave length is that the dielectric between the two high frequency conductors of the M4 coupling system is not homogeneous, as it comprises air and glass layers. It is possible to shift the cut-off wave length of the H mode towards the shortest possible wave length by suitably dimensioning the inner diameter D of the focussing tube. An optimum match of the delay line tothe high frequency line to be coupled is possible in this manner and, simultaneously, the propagation of the H mode is safely prevented.

Furthermore, it has been found to be a very important consideration whether or not the tube is concentrically arranged with respect to the travelling wave tube, i.e., a larger eccentricity favors the formation of the undesired H mode. In view of the fact that during focussing eccentricity cannot be avoided in all instances, the focussing tube is suitably made with a sufficiently large inner diameter D The optimum diameter D with respect to the cut-off wave length of the H coaxial cable mode would result in some instances in undesirably large nonhomogeneities of the field in the vicinity of the short circuit plane. Thus, it is a further object of the invention to provide an annular, inwardly extending or projecting flange inside the focussing tube in the vicinity of the front end of the k/ 4 sleeve at the short circuit plane, said flange being of such size that it produces minimum stray fields, while permitting enough latitude for focussing.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

In the drawings:

FIGURE 1 shows a cross section of an embodiment of the coupling apparatus according to the invention;

FIGURE 2 is an enlarged perspective view of the A/ 4 sleeve according to the invention.

In FIGURE 1, a travelling wave tube 1 has a vacuum envelop, for example, of glass, having an inner diameter D and an outer diameter D A )\/4 sleeve 3 has an outer diameter D and a foccusing tube 6 has an inner diameter D Since the diameters D D and D are fixed in case of a given travelling wave tube, the optimum diameter D of the focussing tube can be calculated with the aid of the above mentioned equation. The focussing tube 6 has annular flanges 7 extending inwardly and constricting the inner cross section of the focussing tube in such a manner, that the proper spacing between the inner and the outer conductors of the A/ 4 coupling system is obtained at these locations for providing the desired short circuits. The high frequency line 2 to be coupled is now coupled to a delay line (not shown) through the two \/4 sleeve 3.

These sleeves 3 are provided with the novel combination of three longitudinal slots 4 and with three trans verse slots running circumferentially in accordance With the invention, as best shown in the perspective illustration of FIGURE 2.

I claim:

1. An apparatus for coupling a high frequency line to a travelling wave tube having a delay line and having a vacuum envelop of dielectric material, and having connected to the high frequency line a metal tube which is located outside of the vacuum envelop and carries focussing means, comprising at least one sleeve means Within the vacuum envelop, said means forming together with the metal tube located outside the vacuum envelop 'a k/ 4 coaxial line producing a high frequency short circuit across one end of the sleeve means, said sleeve means having an odd-numbered plurality of circumferentially disposed slots through its wall opposite the metal tube.

2. An apparatus for coupling a high frequency line to a travelling wave tube having a delay line and having a vacuum envelop of dielectric material, and having connected to the high frequency line a metal tube which is located outside of the vacuum envelop and carries focussing means, comprising at least one sleeve means Within the vacuum envelop, said means forming together with the metal tube located outside the vacuum envelop a 4 coaxial line producing a high frequency short circuit across one end of the sleeve means, said sleeve means having a plurality of circumferentially disposed slots through its wall opposite the metal tube, said slots being located closely to said end of said A/ 4 sleeve means.

3. An apparatus for coupling a high frequency line to a travelling wave tube having a delay line and having a vacuum envelop of dielectric material, and having connected to the :high frequency line a metal tube which is located outside of the vacuum envelop and carries tocussing means, comprising at least one sleeve means within the vacuum envelop, said means forming together with the metal tube located outside the vacuum envelop a 4 coaxial line producing .a high frequency short circuit across one end of .the sleeve means, said sleeve means having a plurality of circumferentially disposed slots through its wall opposite the metal tube, an annular flange of the metal tube extending inwardly towards the envelop adjacent said end of the sleeve means.

4. An apparatus for coupling a high frequency line to a travelling wave tube having a delay line and having a vacuum envelop of dielectric material, and having connected .to the high frequency line a metal tube which is located outside of the vacuum envelop and carries focussing means, comprising at least one sleeve means within the vacuum envelop, said means forming together with the metal tube located outside the vacuum envelop a M4 coaxial line producing a high frequency short circuit across one end of the sleeve means, said sleeve means having a plurality of ci-rcumferentially disposed slots through its Wall opposite the metal tube, said metal tube having an optimum inner diameter substantially represented by the equation:

(1 l in Dz/D wherein:

D is the inner diameter of the dielectric vacuum envelop D2 is the outer diameter of the dielectric vacuum envelop D,, is said inner diameter of the metal tube D is said outer diameter of said \/4 sleeve means, and

E is said dielectric coefiicient of the interposed travelling wave tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,500,417 Kinzer Mar. 14, 1950 2,512,468 Percival June 20, 1950 2,611,102 Bohlke Sept. 16, 1952 2,672,571 Harman Mar. 16, 1954 2,759,156 Younker Aug. 14, 1956 2,799,797 Peter July 16, 1957 2,877,434 Farr et a1. Mar. 10, 1959 FOREIGN PATENTS 944,863 Germany July 19, 1956 

1. AN APPARATUS FOR COUPLING A HIGH FREQUENCY LINE TO A TRAVELLING WAVE TUBE HAVING A DELAY LINE AND HAVING A VACUUM ENVELOP OF DIELECTRIC MATERIAL, AND HAVING CONNECTED TO THE HIGH FREQUENCY LINE A METAL TUBE WHICH IS LOCATED OUTSIDE OF THE VACUUM ENVELOP AND CARRIES FOCUSSING MEANS, COMPRISING AT LEAST ONE SLEEVE MEANS WITHIN THE VACUUM ENVELOP, SAID MEANS FORMING TOGETHER WITH THE METAL TUBE LOCATED OUTSIDE THE VACUUM ENVELOP A $/4 COAXIAL LINE PRODUCING A HIGH FREQUENCY SHORT CIRCUIT ACROSS ONE END OF THE SLEEVE MEANS, SAID SLEEVE MEANS HAVING AN ODD-NUMBERED PLURALITY OF CIRCUMFERENTIALLY DISPOSED SLOTS THROUGH ITS WALL OPPOSITE THE METAL TUBE. 